1. Field of the Invention
This invention relates to a method for forming an improved heat exchange surface. More particularly, a nucleate boiling surface is formed on at least one side of a deformable strip by roll forming. The strip can then be formed into a tube having an enhanced nucleate boiling surface.
2. Background of the Invention
In certain refrigeration applications such as a chiller or an evaporator, the liquid to be cooled is passed through a tube while liquid refrigerant is in contact with the outside of the tube. The refrigerant changes state from a liquid to a vapor absorbing heat from the fluid within the tube. The selection of the external configuration of the tube is extremely influential in determining the boiling characteristics and overall heat transfer rate of the tube. As disclosed in U.S. Pat. Nos. 4,050,507 to Chu et al, 4,474,231 to Staub et al. and 5,054,548 to Zoller, the transfer of heat to a boiling liquid is enhanced by the creation of nucleate boiling sites.
In nucleate boiling, liquid adjacent to a trapped vapor bubble is super heated by the heat exchanger surface. Heat is transferred to the bubble at the liquid vapor interface. The bubble grows in size until surface tension forces are overcome by buoyancy and the bubble breaks free from the surface. As the bubble leaves the surface, fresh liquid wets the now vacated area. The remaining vapor absorbs heat from the fresh liquid to form the next bubble. The vaporization of liquid and continuous stripping of the heated liquid adjacent to the heat transfer surface, together with the convection effect due to the agitation of the liquid pool by the bubbles result in an improved heat transfer rate for the heat exchanger surface.
U.S. Pat. No. 5,054,548, as well as U.S. Pat. Nos. 3,696,861 to Webb and 4,653,163 to Kuwahara et al. disclose manufacturing a nucleate boiling surface on the exterior side of a heat exchange pipe. External fin convolutions are formed on the outside surface of a tube using a fin forming disk. The tip portions of fins are bent toward adjacent fins, producing a substantially confined, elongated space which extends around the outside of the tubing forming a plurality of subsurface channels.
The fins are frequently notched prior to being bent over. When the fins are bent over, the fin tips contact the adjacent fin. The notched portion does not contact an adjacent fin and forms a pore intersecting the subsurface channels. During nucleate boiling, a stream of vapor exits through the pores and fresh liquid flows within the subsurface channels to occupy the space vacated by the streaming vapor. The notched fins may be formed by knurling helical notches on the surface of the tube prior to finning.
A method to form a nucleate boiling surface on the inside wall of a heat exchanger pipe is disclosed in U.S. Pat. No. 5,052,476 to Sukumoda et al. A deformable strip of metal is passed through a sequence of forming rolls. The first roll forms a plurality of U-shaped grooves. The undeformed metal separating the U-shaped grooves forms a plurality of fins. A second forming roll forms a V-shaped notch part way through each fin. Flaring the halves of the split fins towards one another brings adjacent split fins in close proximity. The base of the U-shaped groove forms a subsurface channel with the pore an exit means for vapor in accordance with nucleate boiling practice.
Roll forming has been used to enhance either the inside or outside surface of a strip prior to forming into a tube. For example, U.S. Pat. Nos. 3,861,462, 3,885,622 and 3,906,605 all to McLain and all incorporated by reference in their entireties herein, disclose the use of textured rolls to form a desired pattern on either one or both surfaces of a metallic strip. The strip is formed into a tubular shape by passing through a plurality of tube forming rolls. A welding station then joins longitudinally extending edges of the strip to form a complete tube.
Roll forming has been used to form a continuous groove pattern or diamond shaped pattern as disclosed in the McLain patents. A plurality of discrete roughness elements on the internal surface of a tube in the form of flattened pyramids is disclosed in U.S. Pat. No. 5,070,937 to Mougin et al.
Grooves, diamond shaped patterns and roughness elements increase the surface area of the tube and influence the flow of liquid. However, the patterns do not encourage nucleate boiling. Accordingly, it is an object of the present invention to provide a method for forming a nucleate boiling surface by roll forming. It is a feature of the invention that the nucleate boiling surface is formed by a combination of roll forming and machining operations. It is an advantage of the method of the invention that the nucleate surface is formed prior to conversion of the strip into tube and the nucleate surface may be located on either the inside or outside surface. Another advantage of the invention is that both the size of the subsurface channels and the pores are easily controlled. Yet another advantage is that in addition to forming a nucleate surface by roll forming, the opposing side of the strip may be enhanced. Yet another advantage of the invention is that the strip width is limited only by the capacity of the forming rolls. The strip may be slit to a desired width prior to tube forming.